Mismatch in due dates of pay items of same invoice

Hi,

We have found mismatch in due dates of pay items of same invoice.

We use to run Sales update and usually only one payitem will be generated ie.  original amount (+ve amount) and (-ve amount) will be consilidated and will appear as a single pay item.

We have not found any problem so far.

But suddenly a week before, the invoice amount was split to 2 pay items.  Pay item 1 with a +ve amount, due date more than 45 days from date of invoice.  Pay item 2 with -ve amount, due date as of invoice date.

Have checked all the existing set ups and could not find any error in the set ups.  Payment term for the customer is DD which does not contain any due date.  It is plain.